How many decking boards do I need for a 4m x 4m deck?

For a 16m² deck using standard 145mm boards with a 5mm gap, you will need approximately 32 boards if they are 3.6m long. This includes a small allowance for cuts.

What is the standard gap between decking boards?

In the UK, a gap of 5mm to 8mm is recommended. This allows for the timber to expand and contract with the weather and, more importantly, allows water and debris to fall through, preventing the joists from rotting.

How many screws do I need for decking?

A good rule of thumb is 35 to 40 screws per square metre. You should use two screws at every joist intersection. For a standard 145mm board on 400mm joist centres, this works out accurately.

What is the difference between softwood and composite decking?

Softwood (treated pine) is natural and cheaper but requires annual staining and cleaning. Composite decking is made from recycled wood and plastic; it is more expensive but virtually maintenance-free and doesn't rot or splinter.

How many decking boards do I need for a 4m x 4m deck?

For a 16m² deck using standard 145mm boards with a 5mm gap, you will need approximately 32 boards if they are 3.6m long. This includes a small allowance for cuts.

What is the standard gap between decking boards?

In the UK, a gap of 5mm to 8mm is recommended. This allows for the timber to expand and contract with the weather and, more importantly, allows water and debris to fall through, preventing the joists from rotting.

How many screws do I need for decking?

A good rule of thumb is 35 to 40 screws per square metre. You should use two screws at every joist intersection. For a standard 145mm board on 400mm joist centres, this works out accurately.

What is the difference between softwood and composite decking?

Softwood (treated pine) is natural and cheaper but requires annual staining and cleaning. Composite decking is made from recycled wood and plastic; it is more expensive but virtually maintenance-free and doesn't rot or splinter.

Decking Materials Calculator

Complete material takeoff for timber and composite decks.

Width (m)

Length (m)

Board Width (mm)

Desired Gap (mm)

Purchased Board Length (m)

Material Summary

Waste Tip: Always add 10% to your board count for cuts and wastage. If you are laying the deck diagonally, add 15% as you will have significantly more offcuts.

Decking Design: From Sub-frame to Surface

Outdoor Living Space

A well-built deck is an extension of your home. Whether you choose traditional grooved softwood or modern wood-plastic composite, the success of the project relies on accurate material planning.

Our **decking calculator** handles the complex math of board widths and gap spacing, providing you with a precise linear metre count for your supplier.

Board Specifications

Standard Softwood120mm / 145mm

Standard Composite135mm / 150mm

Max Joist Centres400mm (Standard)

The Golden Rule: Ventilation

The most common cause of deck failure in the UK is poor airflow. If the area under your deck is sealed off, moisture will build up and rot your joists from the inside out within 5 years.

Always ensure there is a gap around the perimeter of the deck to allow air to circulate. If you are 'skirting' the deck, use trellis or leave 10mm gaps between the skirting boards to ensure the sub-frame stays dry.

Screws Matter

Never use standard wood screws. Decking screws are coated to resist the corrosive chemicals in pressure-treated timber. For coastal areas or high-end hardwood decks, always use **Grade 304 or 316 Stainless Steel** screws.

Drainage Fall

Decks should never be perfectly level. Build in a slight fall (approx 1 in 100) away from your house. This prevents water from pooling in the grooves of the boards, reducing the risk of algae growth and slip hazards.

Pro Tip: Joist Tape

Before you screw down your boards, apply **Joist Protection Tape** (Butyl tape) to the top of all your timber joists. This creates a waterproof seal around the screw holes, which are the main points where water enters and rots the sub-frame.

Decking & Sub-frame FAQ

Technical Disclaimer

This calculator estimates surface materials only. It does not calculate the timber required for the sub-frame (joists, posts, and noggins). For structural deck building, always consult UK building regulations regarding spans and load-bearing requirements.